The invention relates to tools for use downhole in, for example, testing formations.
Formation testing tools of this type are designed to take samples of the formation surrounding a drilled hole so that the nature of the formation can be ascertained. In general, such tools are hydraulically operated, hydraulic power being generated by an electric motor, since electrical power is the only feasible source of power which can be carried long distances downhole.
A problem arises on existing formation testing tools in that, if there is a power failure while the tool is being operated, some means of relieving hydraulic pressure used to force, for example, a probe into the surrounding formation is needed otherwise the tool will be permanently jammed in the hole.
Hitherto, formation testing tools have provided a hydraulic dumping chamber to relieve hydraulic pressure, but once dumped, the system cannot be reset without bringing the tool to the surface. This is clearly inconvenient and costly in both time and money.